While Sputnik 1 was the first spacecraft to orbit the Earth, other man-made objects had previously reached an altitude of 100 km, which is the height required by the international organization "Fédération Aéronautique Internationale to count as a spaceflight. This altitude is called the "Kármán line. In particular, in the 1940s there were "several test launches of the "V-2 rocket, some of which reached altitudes well over 100 km.

As of 2016, only three nations have flown manned spacecraft: USSR/Russia, USA, and China. The first manned spacecraft was "Vostok 1, which carried Soviet cosmonaut "Yuri Gagarin into space in 1961, and completed a full Earth orbit. There were five other manned missions which used a "Vostok spacecraft.[11] The second manned spacecraft was named "Freedom 7, and it performed a "sub-orbital spaceflight in 1961 carrying American astronaut "Alan Shepard to an altitude of just over 187 kilometers (116 mi). There were five other manned missions using "Mercury spacecraft.

Some reusable vehicles have been designed only for manned spaceflight, and these are often called spaceplanes. The first example of such was the "North American X-15 spaceplane, which conducted two manned flights which reached an altitude of over 100 km in the 1960s. The first reusable spacecraft, the "X-15, was air-launched on a suborbital trajectory on July 19, 1963.

The first partially reusable orbital spacecraft, a winged non-capsule, the "Space Shuttle, was launched by the USA on the 20th anniversary of "Yuri Gagarin's flight, on April 12, 1981. During the Shuttle era, six orbiters were built, all of which have flown in the atmosphere and five of which have flown in space. "Enterprise was used only for approach and landing tests, launching from the back of a "Boeing 747 SCA and gliding to deadstick landings at "Edwards AFB, California. The first Space Shuttle to fly into space was "Columbia, followed by "Challenger, "Discovery, "Atlantis, and "Endeavour. Endeavour was built to replace Challenger when it was "lost in January 1986. Columbia "broke up during reentry in February 2003.

The first automatic partially reusable spacecraft was the "Buran-class shuttle, launched by the USSR on November 15, 1988, although it made only one flight and this was unmanned. This "spaceplane was designed for a crew and strongly resembled the U.S. Space Shuttle, although its drop-off boosters used liquid propellants and its main engines were located at the base of what would be the external tank in the American Shuttle. Lack of funding, complicated by the "dissolution of the USSR, prevented any further flights of Buran. The Space Shuttle was subsequently modified to allow for autonomous re-entry in case of necessity.

A spacecraft system comprises various subsystems, depending on the mission profile. Spacecraft subsystems comprise the spacecraft's ""bus" and may include attitude determination and control (variously called ADAC, ADC, or ACS), guidance, navigation and control (GNC or GN&C), communications (comms), command and data handling (CDH or C&DH), power (EPS), "thermal control (TCS), propulsion, and structures. Attached to the bus are typically "payloads.

Life support

Spacecraft intended for human spaceflight must also include a "life support system for the crew.

A Spacecraft needs an "attitude control subsystem to be correctly oriented in space and respond to external "torques and forces properly. The attitude control subsystem consists of "sensors and "actuators, together with controlling algorithms. The attitude-control subsystem permits proper pointing for the science objective, sun pointing for power to the solar arrays and earth pointing for communications.

GNC

Guidance refers to the calculation of the commands (usually done by the CDH subsystem) needed to steer the spacecraft where it is desired to be. Navigation means determining a spacecraft's "orbital elements or position. Control means adjusting the path of the spacecraft to meet mission requirements.

Command and data handling

The CDH subsystem receives commands from the communications subsystem, performs validation and decoding of the commands, and distributes the commands to the appropriate spacecraft subsystems and components. The CDH also receives housekeeping data and science data from the other spacecraft subsystems and components, and packages the data for storage on a "data recorder or transmission to the ground via the communications subsystem. Other functions of the CDH include maintaining the spacecraft clock and state-of-health monitoring.

Spacecraft, both "robotic and "crewed, utilize various communications systems for communication with terrestrial stations as well as for communication between spacecraft in space. Technologies utilized include "RF and "optical communication. In addition, some spacecraft payloads are explicitly for the purpose of ground–ground "communication using "receiver/retransmitter electronic technologies.

Power

Spacecraft need an electrical power generation and distribution subsystem for powering the various spacecraft subsystems. For spacecraft near the "Sun, "solar panels are frequently used to generate electrical power. Spacecraft designed to operate in more distant locations, for example "Jupiter, might employ a "radioisotope thermoelectric generator (RTG) to generate electrical power. Electrical power is sent through power conditioning equipment before it passes through a power distribution unit over an electrical bus to other spacecraft components. Batteries are typically connected to the bus via a battery charge regulator, and the batteries are used to provide electrical power during periods when primary power is not available, for example when a low Earth orbit spacecraft is "eclipsed by Earth.

Thermal control

Spacecraft must be engineered to withstand transit through "Earth's atmosphere and the "space environment. They must operate in a "vacuum with temperatures potentially ranging across hundreds of degrees "Celsius as well as (if subject to reentry) in the presence of plasmas. Material requirements are such that either high melting temperature, low density materials such as "beryllium and "reinforced carbon–carbon or (possibly due to the lower thickness requirements despite its high density) "tungsten or "ablative carbon–carbon composites are used. Depending on mission profile, spacecraft may also need to operate on the surface of another planetary body. The "thermal control subsystem can be passive, dependent on the selection of materials with specific radiative properties. Active thermal control makes use of electrical heaters and certain "actuators such as louvers to control temperature ranges of equipments within specific ranges.

Spacecraft may or may not have a "propulsion subsystem, depending on whether or not the mission profile calls for propulsion. The "Swift spacecraft is an example of a spacecraft that does not have a propulsion subsystem. Typically though, LEO spacecraft include a propulsion subsystem for altitude adjustments (drag make-up maneuvers) and "inclination adjustment maneuvers. A propulsion system is also needed for spacecraft that perform momentum management maneuvers. Components of a conventional propulsion subsystem include fuel, tankage, valves, pipes, and "thrusters. The thermal control system interfaces with the propulsion subsystem by monitoring the temperature of those components, and by preheating tanks and thrusters in preparation for a spacecraft maneuver.

Structures

Spacecraft must be engineered to withstand launch loads imparted by the launch vehicle, and must have a point of attachment for all the other subsystems. Depending on mission profile, the structural subsystem might need to withstand loads imparted by entry into the "atmosphere of another planetary body, and landing on the surface of another planetary body.

Payload

The payload depends on the mission of the spacecraft, and is typically regarded as the part of the spacecraft "that pays the bills". Typical payloads could include scientific instruments ("cameras, "telescopes, or "particle detectors, for example), cargo, or a "human crew.

The "ground segment, though not technically part of the spacecraft, is vital to the operation of the spacecraft. Typical components of a ground segment in use during normal operations include a mission operations facility where the flight operations team conducts the operations of the spacecraft, a data processing and storage facility, "ground stations to radiate signals to and receive signals from the spacecraft, and a voice and data communications network to connect all mission elements.[14]